Buff Master

Buff Master is a skill under the Specials Skill Tree.
It has a dependency on Brew Master, meaning that at least 10 points must be placed into Brew Master before you can have any points in Buff Master.
It is available to players of level 500 or higher.
Here's the official description :

0.2% per point chance to half the stamina cost (rounding up) when casting skills on other players. (Does not work on self!)

First to note, that does not mean that a player cannot cast the Buff Master buff on himself.
It means that the chance to half the stamina cost does not apply when the player with the buff cast skills on himself.
Since the skill is available to level 500 players, it is safe to assume that players will this skill will have it at the maximum level (which is level 175, with maxed Fury Casting).
That means there is a 35% chance to half the stamina cost.
Ignoring rounding, it is saving 17.5% of the stamina.

The buff itself cost 10 stamina and has a duration of 60 minutes (90 minutes with maxed sustain).
So if you are a level 500 or higher with the buff, and casting more than 60 stamina of skills on other players (within the duration of the Buff Master), you should first cast this buff on yourself.

If you need to buy this buff, then you have to figure out how much your stamina worth, vs how much are players charging you for this buff. The same ratio of 17.5% savings apply if you buy a level 175 buff. That means, if you will be using 100 stam in buffs (on others), you will be saving 17.5 stam. If you will be using 150 stam in buffs, you will be saving 26.25 stam (which is a little more than 1 fsp worth in stam). The going rate at the time of writing this blog appears to be 1 fsp to get this buff from a level 500 player.
There is also a Potion of Skills Master which you can buy from the Auction House.
That potion has a minimum price of 25,000 gold. Which I have often seen it as not being purchased at all. Maybe things will change after players read this blog, or maybe not.
That potion is a skill level 100 with a duration of 60 minutes.
That means it is a 10% savings in stamina.
So now, consider the following ::

100 stams used in buffing others
Option 1 : Buff yourself, 17.5 stam saved on average, 10 stam used on buff, 7.5 stam total savings.
Option 2 : Buy the buff, 1 fsp. 17.5 stam saved on average. (Not good, better to use the fsp to buy 25 stam).
Option 3 : Buy the potion, 25k. 10 stam saved on average. (Good deal, probably better than option 1 even for level 500 players).

150 stams used in buffing others
Option 1 : Buff yourself, 26.25 stam saved on average, 10 stam used on buff, 16.25 stam total savings.
Option 2 : Buy the buff, 1 fsp. 26.25 stam saved on average. This is better than using the fsp to buy stam.
Option 3 : Buy the potion, 25k, 15 stam saved. (This is a better savings, since it is cheaper than 100k/25stam rate. Level 500 players are probably better off using option 1, saving more stam without paying the 25k).

250 stams used in buffing others
Option 1 : Buff yourself, 43.75 stam saved, 10 stam used, 33.75 stam total savings.
Option 2 : Buy the buff, 1 fsp. 43.75 stam saved on average. (Assuming rate of 1 fsp = 25 stam, you saved 18.75 stam).
Option 3 : Buy the potion, 25k, 25 stam saved on average. (Assuming rate of 100k = 25 stam, 25k = 6.25 stam, you saved 18.75 stam) (This is the breakeven point if you use the current buff buying rate vs potion rate, as well as stamina valuation, and that means above 250 stams used, option 2 is probably the best option for players below level 500 or without the skill).

What if the price changes?
What if I value stamina differently?
This is especially true since it is 25 stamina is really worth less than 1 fsp (or 1 stamina is worth less than 4k). Players still buy stamina with fsp, but usually only when they are fully buffed. It makes sense to save the stamina and then to use it all in a bigger block. It is this same argument that players hunt with full stamina insted of daily. Consider selling buffs at 1 fsp per 25 stamina, that's like banking your stamina into a reserve, as you can buy the stamina at a more convenient/strategic time. Now, if the stamina bank allows you to withdraw only 25 stamina for every 30 stamina banked, will you do it? Maybe. But if you will, then the value of stamina just went down to 3.3k. Everyone will measure this differently, since at different stage of the game, you may want more fsp (for upgrades, equipment or otherwise), or more stam (when hunting and so close to finishing off that tough level and you're all buffed up).

So here are the variables :
Value of stamina = V(measured as gold per stamina, above examples were using 4k/stam)
Price to get the level 175 buff = B (in gold)
Price to buy the level 100 potion = P (in gold)
Amount of stamina used on others = S

Option 1 : 10 stamina used, .175 * S saved. Total savings = (.175 * S - 10)*V
Option 2 : Buy the buff, B. .175 * S saved. Total savings = .175 * S * V - B
Option 3 : Buy the potion, P. .1 * S saved. Total savings = .1 * S * V - P

Now you can plug in your own values and decide which option you use. Option 1 may not be available to everyone. Have fun buffing.